Brake fluid

Car Mechanics (UK) - - Help! -

I asked the me­chanic at my lo­cal MOT test cen­tre if I should get the brake fluid re­placed on my car at 44,000 miles. He said it wasn’t nec­es­sary with mod­ern brake fluid un­less the sys­tem has to be bled af­ter, say, fit­ting a new caliper. Do you agree? C Beck

The im­por­tance of brake fluid is of­ten over­looked un­til the mo­ment it fails. It is hy­dro­scopic, which means it will ab­sorb mois­ture from the air. If it is in a sealed sys­tem, the the­ory is that it will not ab­sorb mois­ture be­cause it’s not be­ing ex­posed to the at­mos­phere. This is the rea­son why some peo­ple con­sider re­plac­ing the fluid is an un­nec­es­sary process.

Many years ago, when I worked on po­lice ve­hi­cles, we would reg­u­larly re­place the brake fluid for an­other rea­son. Ex­ten­sive heat­ing of the brake fluid un­der harsh brak­ing mean the fluid’s con­di­tion and sta­bil­ity will de­te­ri­o­rate, caus­ing any mois­ture present to boil. While spe­cial­ist sil­i­cone flu­ids which are hy­dropho­bic will re­pel mois­ture in the sur­round­ing air, most brake flu­ids are gly­col-based and will ab­sorb mois­ture re­gard­less of which DOT rat­ing they have for boil­ing point.

When re­plac­ing brake fluid, it is im­por­tant to only use fluid from a sealed con­tainer. Work­shops nor­mally have brake fluid stored in 20-litre sealed bags and dis­pensed by a tap to pre­vent the pos­si­bil­ity of wa­ter/mois­ture ingress. Sil­i­cone flu­ids are more com­press­ible than gly­col flu­ids and can give a spongy feel to the brake pedal. They should never be mixed with gly­col flu­ids and are not con­sid­ered a re­place­ment for mod­ern ve­hi­cles, but are a suit­able al­ter­na­tive for clas­sic cars.

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